1. Field of the Invention
This invention relates to improved packed bed apparatus and to the process for making such apparatus. More particularly, the invention relates to improvements in packed bed apparatus in which the packing material has been triaxially compressed.
2. Description of the Prior Art
Liquid chromatography is a process utilized in both preparative and analytical chemistry. Essentially, the process comprises the interacting of a mobile phase with a stationary phase. Typically, the stationary phase is a surface active powder such as silica, alumina, an inert size-separating material such as a gel-permeation chromatography packing, or the like. This powder is contained within a chromatographic column. The mobile phase, which generally consists of a carrier fluid and a sample of a chemical to be identified, analyzed or purified, is passed through the column. The process is typically utilized to separate various chemical compounds in an unknown sample. This separation is made by using a stationary phase which differentially retards the progress of the different components of the sample through the column so that these components are separated and leave the column at different times. The separation may also be achieved by an exclusion process based on the difference in sizes between molecules, for example by gel-permeation chromatography processes.
In order to achieve separation of sample components which are very close to each other in chemical and physical properties, highly sophisticated procedures have been developed in the many processing techniques associated with liquid chromatography. Special pumps and valves have been developed for presenting samples to the inlets of the chromatographic columns with as much integrity as is possible to avoid building into the process an initial and inherent dispersal of the sample which dispersal would tend to reduce the resolution capabilities of the packing within the chromatographic column. Much work has been done to provide flow-distributing devices at the inlets to the columns to assure the even placement of the samples across the cross-sectional areas of the columns. Also, a great deal of technical effort has provided improved chromatographic packings and highly sophisticated analytical apparatus for measuring various properties of the liquid effluent leaving the column.
Despite such work as has been described above, it has remained a problem to achieve an uniform packing of the chromatographic material within a column. Many techniques have been suggested including vibration. See, e.g. U.S. Pat. No. 3,300,849. All of these techniques require careful control if segregation of particles by size is to be avoided and uniformly packed columns are to be obtained. Even after the column is filled, problems exist in maintaining the packing in proper condition during transportation and operation of the packed columns. See U.S. Pat. No. 3,349,920.
In general, the most commonly used practice of filling a high-performance column has been a costly method including slurrying the packing and passing the slurry into the column, thereby using the column itself as a form for placing a closely packed bed of chromatographic packing therein. Despite this costly time-consuming method of column manufacture, shifting of the packing may occur during shipment if the column is subjected to various vibration and other transient non-predictable physical abuse. Shifting tends to cause voids in the packing within the column and such voids can wholly destroy the separation capability of a column. Such defects in columns with stainless steel walls are not usually detectable until a standard sample is measured as a control. Suppliers of quality chromatographic columns have individually pre-tested each column before shipment to the customer to assure that the packing was properly placed in the column. The procedure, however, provides no protection against the hazards encountered during shipment or during use by the customer.
A number of solutions have been suggested for holding the packing in place. Some of these, like the aforementioned vibration technique and slurrying technique, emphasize a maximum effort to put a conventional packing into a column in such a way as to have it assume a stable position. Other techniques such as those described in U.S. Pat. No. 3,808,125 use rather complex or expensive procedures for fastening the packing to the column wall.
None of these attempts by the prior art have been dependably successful in achieving consistently excellent chromatographic performance and long term bed stability from column to column at a cost which could make the apparatus available to the broadest spectrum of chromatographers.
Although the foregoing description of problems relating to chromatographic columns has been largely devoted to liquid chromatographic columns, it is emphasized that many of the problems described above also relate to gas chromatography, i.e. chromatography wherein the sample and mobile phase are in gaseous, rather than liquid, form. In many respects, the problems relate to all packed-bed apparatus comprising a porous mass of particles intended to be intimately and uniformly contacted by a fluid. Such apparatus includes catalytic beds for the treating of gas and liquid, packed beds used in ion exchange processes, in electrophoresis applications, and the like. It is intended that the invention described below be viewed as an improvement in packed-column preparation for all such processes; albeit, the invention will be seen to have particular advantage in the field of liquid chromatography.
In discussing column packing processes, it is helpful to recognize four kinds of space, all of which can be referred to as "void volume." These include (1) void volume inside a porous particle; (2) theoretical void volume between particles, i.e. the type of unavoidable volume which would result from a perfectly packed bed of spheres of the same size; (3) void volume between particles which is attributable to imperfect packing of particles, usually present to some extent in any actual packed structure and (4) void volume which represents relatively large voids resulting from the consolidation of those voids described in (3). Voids (4) substantially reduce resolution of a sample being subjected to chromatographic analysis.
The invention disclosed below is believed to be most useful in avoiding the occurrence of such void volume as described in (4). The present invention also tends to reduce void volume as described in (3); moreover, it makes such void volume more nearly uniform, and closer to the theoretical ideal (2). Void volume, as generally used herein relates to a composite of void volumes (3) and (4).
Some workers have suggested comression of the packing of a chromatographic column by force directed longitudinally, i.e. parallel to the direction of liquid flow. However, such a procedure is relatively ineffective probably because the packing tends to form bridges which interfere with propagation of the compression force downwardly throughout the length of the column. An example of longitudinal compression is described in the Journal of Chromatographic Science of October, 1974, in an article entitled "Description and Performance of an 8 cm i.d. Column For Preparative Scale High Pressure Liquid-Solid Chromatography" by Godbille and Devaux.
In copending U.S. application Ser. No. 848,752 filed Nov. 4, 1977, which is a continuation of Ser. No. 638,301 filed Dec. 8, 1975, we have described in detail a method of making chromatographic columns which obviates the difficulties encountered by the prior art methods discussed above and provides consistently uniformly packed chromatographic columns having improved quality and uniformity of separation characteristics from column to column. In the chromatographic columns disclosed in the application the packing material is radially compressed by applying a force predominantly in a direction which is normal to the direction of flow of the liquid through the column and to the longitudinal dimension of the packing. It is suggested that such radial compression may be achieved by mechanical and fluid means. In one embodiment a polymeric walled column is placed in a packing chamber and expanded, for example by air pressure and heating, filled with packing material and then radially compressed, for example by air pressure or by allowing the polymeric wall to cool and contract. Since shipment of the column may loosen the radial force on the packing, the column should be recompressed upon receipt by the user. This application discusses in detail numerous other ways in which the chromatographic packing can be radially compressed and therefore is incorporated herein by reference as though set out in its entirety herein.
U.S. Pat. No. 3,570,673 discloses a method for producing a separation column for use in liquid chromatography consisting of inserting a bundle of parallel inorganic fibers into a tube of chemically resistant material, shrinking the tube onto the bundle of fibers, and then forcing a solvent through the bundle to remove the soluble phase of the fibers and obtaining a porous fibrous body.